Plasma spray method for the manufacturing of an ion conducting membrane and an ion conducting membrane

ABSTRACT

A plasma spray method for the manufacture of an ion conducting membrane, in particular of a hydrogen ion conducting membrane or of an oxygen ion conducting membrane is suggested. In which method the membrane is deposited as a layer ( 11 ) on a substrate ( 10 ) in a process chamber, wherein a starting material (P) is sprayed onto a surface of the substrate ( 10 ) by means of a process gas (G) in the form of a process beam ( 2 ). The starting material is injected into a plasma at a low process pressure which is at most 10000 Pa and is partially or completely melted there. In accordance with the invention the substrate ( 10 ) has pores ( 30 ) which are connected amongst one another so that the substrate ( 10 ) is gas permeable and a portion of an overall pore area of an overall area of the coating surface ( 31, 131 ) amounts to at least 30%, in particular to at least 40%.

The invention relates to a plasma spray method for the manufacture of anion conducting membrane, in particular of a hydrogen ion conductingmembrane or of an oxygen ion conducting membrane in accordance with thepreamble of claim 1 and to an ion conducting membrane, in particular toa hydrogen ion conducting membrane or an oxygen ion conducting membranein accordance with the preamble of claim 15.

Ion conducting membranes are membranes which have a high selectivepermeability for specific ions. Oxygen permeable membranes are layerswhich have a high selective permeability for oxygen or oxygen ions andare substantially impermeable for other gases or ions. Correspondinglysuch membranes are used in order to extract or to purify oxygen from gasmixtures or from fluid mixtures. The same is true for hydrogen permeablemembranes for the extraction of hydrogen from gas mixtures or from fluidmixtures.

Such membranes can be manufactured from different materials they can,for example, be composed of complex oxide materials which have aspecific chemical composition and form specific phases. In particularceramic membranes are known which are composed of oxides of theperovskite type and which are manufactured in the form of thin,dense—this means non-porous—layers. Such membranes, for example, haveboth an ion conductivity for oxygen or hydrogen and also have anelectron conductivity.

A material which is investigated and used today for the manufacture, inparticular of oxygen permeable membranes is a ceramic material which hasa perovskite structure and includes the elements lanthanum (La),strontium (Sr), cobalt (Co) and iron (Fe) besides oxygen. The substanceis typically referred to as LSCF in accordance with the respective firstletter of these four elements.

Oxygen permeable or hydrogen permeable membranes or generally ionconducting membranes of such materials can, for example, be manufacturedby means of conventional manufacturing techniques for ceramics, such as,for example, pressing, tape casting, slip casting or sintering or alsoby means of thermal spraying. In particular thermal spray processes aresuitable for the latter which are carried out in a vacuum, thistypically means that the spray process is carried out at a processpressure which is smaller than the environmental pressure (normal airpressure).

A thermal low pressure plasma spray process or a vacuum plasma sprayprocess is in particular suitable which is referred to as LPPS method(low pressure plasma spraying). By means of this vacuum plasma spraymethod particularly thin and dense layers can be sprayed particularlywell, i.e. such layers which are also required for an ion conducting oroxygen permeable or hydrogen permeable membrane.

In this connection the ion conducting membrane is deposited as a layeron a substrate in a process chamber. For this purpose, starting materialis sprayed onto a surface of the substrate by means of a process gas inthe form of a process beam. The starting material is injected into aplasma at a low process pressure which, for example, is at most 10 000Pa and is partially or completely melted there.

During an LPPS method the ion conducting membrane is deposited on asubstrate in the form of a layer. The substrate generally serves thepurpose of supporting the thereby arising very thin and brittle layerand to thus make it manageable.

For this reason, it is the object of the invention to provide a plasmaspray process with which a manageable combination of substrate and ionconducting membrane, in particular a hydrogen ion conducting membrane oran oxygen ion conducting membrane, can be manufactured which moreoverenables an effective extraction, in particular of hydrogen or of oxygenfrom gas mixtures or from fluid mixtures. It is furthermore the objectof the invention to provide a resistant combination of ion conductingmembrane, in particular of a hydrogen ion conducting membrane or of anoxygen ion conducting membrane on a metallic porous substrate whichmoreover enables an effective extraction, in particular of hydrogen orof oxygen from gas mixtures or from fluid mixtures.

In accordance with the invention this object is satisfied by a plasmaspray method in accordance with claim 1 and by an ion conductingmembrane on a substrate in accordance with claim 15.

In accordance with the invention the substrate has pores which areconnected amongst one another so that the substrate is gas permeable anda portion of an overall pore area of an overall area of the coatingsurface amounts to at least 30%, in particular to at least 40%, thus,for example, 40% or 45%. The sum of all surfaces of the open pores ofthe coating surface of the substrate should be understood to be theoverall pore surface. The gas mixture from which, in particular hydrogenor oxygen should be extracted is guided to the open pores at the coatingmembrane and comes into contact there with the membrane from a backsubstrate side lying remote from the coating surface via the poresconnected amongst one another. The hydrogen or oxygen ions can thenpermeate through the membrane starting from the open pores and thushydrogen or oxygen can be extracted from the gas mixture.

Thereby the substrate can also guide the fluid mixture or the gasmixture from which the hydrogen or oxygen should be extracted to themembrane in addition to supporting the membrane. Furthermore, it isensured by the mentioned lower boundary of the overall pore surface thata large portion of the membrane also comes into contact with the fluidmixture or the gas mixture and thus that an effective extraction ofhydrogen or oxygen is also ensured.

The overall pore area can, for example, be determined thereby that thepores of the coating surface are colored in and the area of the coloredin pores is determined by an optical measurement process.

In an embodiment of the invention the pores have a mean pore size of atleast 1 micrometer, with this statement, in particular not only relatingto the pores at the substrate surface, but also to the pores in theinterior of the substrate. The mean pore size is, in particulardetermined thereby that, as described above, the open pores resultingfrom a cut through the substrate are colored in. In order to determinethe mean pore size a straight line is subsequently placed onto the cut.Then the individual sections of the mentioned line, which sections lieon the pores are measured and a mean value is formed from the measuredlengths. This mean value corresponds to the mean pore size.

Investigations have shown that the through-flow is strongly hinderedwhich results in a worse transport of the gas mixture to the membranefor pores having a pore size smaller than 1 micrometer.

In an embodiment of the invention the substrate has a useful porosity ofat least 20%, in particular of at least 30%, thus, for example, 30% or40% with respect to an overall volume of the substrate. The usefulporosity results from the quotient of a hollow space volume connectedamongst one another to the overall volume of the substrate. The thusso-called closed hollow spaces which have no connection to theenvironment of the substrate are not of interest here. The open hollowspaces result in the pores at the coating surfaces of the substrate viawhich pores the gas mixture or fluid mixture can come into contact withthe membrane.

The gas mixture or the fluid mixture is particularly well guided to themembrane which enables a particularly effective extraction of hydrogenor of oxygen from gas mixtures or fluid mixtures due to the use of asubstrate having the mentioned porosity.

An estimation of the porosity can also take place starting from theabovementioned determination of the overall pore area. On the assumptionthat the overall pore area is approximately equal in each of the layersparallel to the coating surface the porosity can thereby be calculated.

The useful porosity can, for example, be determined thereby that it isdetermined which volume of a gas or a fluid the substrate can take up.The useful porosity can then be determined from the ratio of thusdetermined volume to the overall volume of this substrate.

The substrate is in particular manufactured by means of a sinteringmethod. Thereby a free shaping of the substrate is enabled and aparticularly high porosity of the substrate can furthermore be achieved.

In an embodiment of the invention micro-passages are introduced into thesubstrate before or after the coating for the improvement of a gas glowpossibility in the direction of a coating surface. The micro-passages inparticular have a diameter of between 5 and 150 micrometers and aretypically introduced into the substrate by means of a laser drillingmethod. Thereby a particularly effective transport of the gas mixture orof the fluid mixture to the membrane is possible.

The micro-passages are in particular orientated in the direction of thecoating surface from a back substrate side lying remote form the coatingsurface and end before the coating surface. Thereby, the gas mixture orfluid mixture which is guided via the back substrate side is, on the onehand, very effectively guided to the membrane. On the other hand, it isprevented through the ending before the coating surface that too largerecesses arise at the coating surface which, as described above,negatively influence the quality of the layer and thus of the membranetoo strongly.

It is also possible that the micro-passages reach up to the coatingsurface and therefore quasi form a pore. The introduction then takesplace, in particular prior to the coating so that the micro-passages arecovered by the membrane.

Since the membrane with the substrate should be used in particular alsoin highly corrosive environments and at high temperatures of above 500°C., the iron alloy should include chromium, whereby a high resistance tocorrosion can be achieved.

In an embodiment of the invention it is therefore suggested that thesubstrate, on which the layer forming the membrane is deposited, ismanufactured from an iron alloy which has a chromium portion which islarger than 20 weight percent and in particular larger than 25 weightpercent, i.e. for example, 22 or 30 weight percent.

It is thereby achieved that the substrate and thus also the combinationof substrate and ion conducting membrane is very resistant and inparticular corrosion resistant. At the same time the membranemanufactured by means of the plasma spray method has a good ionconductivity, whereby an effective extraction of hydrogen or oxygen fromgas mixtures or from fluid mixtures is enabled.

Beside chromium the iron alloy can in particular also include carbon.Furthermore, also further constituents such as, for example, cobalt,manganese, molybdenum, niobium, vanadium or tungsten can be present.

The substrate can, for example, be made of an alloy of 47% nickel (Ni),22% chromium (Cr), 18% iron (Fe), 9% molybdenum (Mo), 1.5% cobalt (Co),0.6% tungsten (W), 0.1% carbon (C), 1% manganese (Mn), 1% silicon (Si)and 0.008% boron (B).

The chromium included in the substrate can, however, lead to problems oncarrying out the method at too high temperatures of the substrate.Chromium particles can then easily arrive in the coating surface andreact to chromium oxide there. Thereby a layer can arise which hindersan ion exchange between gas and membrane. Furthermore, chromium oxidecannot conduct any electrons. However, excess electrons arise during useof the membranes which electrons then have to be guided away from thesubstrate. In this respect, a chromium oxide layer would likewise be ofhindrance.

In the method in accordance with the invention relatively hightemperatures arise at the substrate due to the working principle. Thisis true for the coating process itself, but is also true for a heatingphase of the substrate which takes place prior to the actual coating. Inthis respect, a formation of the described chromium oxide layer can bebrought about. On the other hand, however, also the danger is presentthat the structure of the substrate is damaged by too high temperatures.These damages are referred to as so-called “creeping”.

In an embodiment of the invention process parameters are set so that atemperature of the substrate amounts to between 250 and 850° C. duringthe deposition of the layer. In this connection in particular a processenthalpy, a spacing of the substrate to a plasma torch and a period oftime in which the process beam is applied to the substrate withoutinterruption or a frequency of the application with the process beam areto be understood as process parameters.

Dense layers can be generated in a comparatively short time in thementioned temperature range which is required for the use of the layeras a membrane.

The temperature of the substrate can be measured during the process bymeans of a pyrometer known per se. In this connection, the heatradiation emitted by the substrate is measured and evaluated. Themeasured temperatures can be used for a regulation of the processparameters. However, it is also possible that the temperatures aredetermined in a test phase and that the required process parameters formaintaining the temperature boundaries are determined in this testphase. The so determined process parameters can then be used for asubsequent application of the method and the temperature boundaries canthus be maintained.

The described damages of the substrate through “creeping” and/or throughthe formation of chromium oxide are not only dependent on the absolutetemperature, but also on the duration of how long the temperature ispresent.

For this reason, the process parameters are set, in particular so that atemperature of the substrate is only higher than the temperatureboundary for a maximum period of time during the carrying out of themethod. The maximum temperature and the maximum period of time arestrongly dependent on the material of the substrate. On the use of ametallic substrate, for example, having the above-mentioned composition,the maximum temperature amounts to in particular 800° C. and the maximumperiod of time in particular amounts to 5 minutes. Furthermore, theabove-mentioned temperature range of 250 to 850° C. is still true forthe maximum temperature.

Preferably an inert atmosphere or an atmosphere with reduced oxygencontent is present during the spraying in the process chamber.

The membrane preferably also has an electron conductivity besides itsion conductivity.

The plasma spray process is preferably carried out so that the plasmadefocuses and accelerates the process beam. Particularly good thin anddense layers can be manufactured by means of this method.

In practice, it has been found advantageous when the process pressure inthe process chamber is set to a value of at least 50 Pa and to at most2000 Pa.

In a preferred embodiment, the layer forming the membrane is composed ofa ceramic material which is an oxide of the perovskite type.

The method is in particular carried out so that the starting material(precursor) is a powder whose chemical composition is substantially thesame as the chemical composition of the layer, this means that a powderis used as a starting material which substantially has the same chemicalcomposition which the layer sprayed should also have.

With regard to the oxygen permeability it has been advantageously shownthat the layer should be composed of a perovskite which includeslanthanum (La), strontium (Sr), cobalt (Co) and iron (Fe). In thisconnection the term “composed of” is understood to mean that thesubstantial portion of the layer is present in the form of a perovskitephase. Naturally, it is also possible that also other phases are presentto a lesser degree in this layer.

For the manufacture of in particular hydrogen permeable membranes, forexample, a ceramic having a perovskite structure can be used whichincludes the elements barium (Ba), zirconium (Zr), cerium (Ce), yttrium(Y), ytterbium (Yb) and europium (Eu) or lanthanium (La), strontium(Sr), chromium (Cr), yttrium (Y) and aluminum (Al).

Preferably, the plasma spray method is carried out so that the layergenerated on the substrate has a thickness of less than 150 micrometersand preferably of 20 to 60 micrometers. This layer thickness has beentried and tested for oxygen permeable membranes or for hydrogenpermeable membranes.

In practice it has been tried and tested that the overall flow rate ofthe process gas on plasma spraying is smaller than 200 SLPM and inparticular amounts to 100 to 160 SLPM (SLPM: standard liter per minute).

For a first preferred embodiment of the method the process gas is amixture of argon and helium.

For a second preferred embodiment of the method the process gas iscomposed of argon, helium and hydrogen.

It has also been found advantageous, when the process beam is pivoted orrastered (scanned) relative to the surface of the substrate. This can,for example, take place by the pivoting of the plasma generator and/orof the plasma source and/or of the exit nozzle. The process beam is thusguided relative to the substrate so that the substrate is rastered, i.e.is covered once or a plurality of times by the process beam.Alternatively, or in addition, it is naturally also possible to move thesubstrate. Naturally many possibilities are possible to realize thisrelative movement between the process beam and the substrate. This pivotmovement and/or rastering of the substrate has the effect that theoxygen introduced into the process chamber comes into contact with theprocess beam or with the layer being formed on the substrate in an asgood as possible manner.

An ion conducting membrane on a metal porous substrate, in particular anoxygen permeable membrane or a hydrogen permeable membrane is furtherprovided by the invention, which membrane is deposited as a layer on thesubstrate by means of a plasma spray method in a process chamber,wherein a starting material in the form of a process beam is sprayedonto a surface of a substrate by means of a process gas, with thestarting material being injected at a low process pressure, which is atmost 10 000 Pa and is partially or completely melted there and thesubstrate is manufactured from a metal alloy which has a chromiumproportion of between 5 and 20 weight percent.

Further advantageous measures and preferred embodiments of the inventionresult from the dependent claims.

In the following, the invention will be explained in detail by means ofembodiments and with reference to the drawings. In the schematic drawingthere is partially shown in section:

FIG. 1 a schematic illustration of an apparatus for carrying out amethod in accordance with the invention; and

FIG. 2 a schematic illustration of a substrate having a layer depositedon the substrate;

FIG. 3 a detail of the substrate having the layer;

FIG. 4 a detail of the substrate in a top view; and

FIG. 5 a second embodiment of the substrate having a layer.

The plasma spray method for the manufacture of an ion conductingmembrane in accordance with the invention will be explained in thefollowing with reference to the case of application particularlyrelevant to practice, in which the membrane is a permeable membraneselective for oxygen which thus has an ion conductivity for oxygen. Themembrane further preferably also has an electron conductivity. Themethod is a thermal spray method which is carried out in vacuum, thus ata process pressure which is smaller than the environmental pressure.

FIG. 1 shows a plasma spray apparatus in a very schematic illustrationwhich is totally referred to with the reference numeral 1 and issuitable for carrying out a method in accordance with the invention. Ametallic porous substrate 10 is furthermore illustrated in FIG. 1schematically, onto which an oxygen permeable membrane in the form oflayer 11 is deposited. A process chamber 12 is further indicated inwhich the method is carried out.

The method in accordance with the invention includes a plasma sprayingwhich is described in kind in WO-A-03/087422 or also in U.S. Pat. No.5,853,815. This plasma spray method is a thermal spraying for themanufacture of a so-called LPPS thin film (LPPS=Low Pressure PlasmaSpraying).

A LPPS-based method is specifically carried out with the plasma sprayapparatus 1 illustrated in FIG. 1. During this a conventional LPPSplasma spray method is changed from a process point of view, wherein aspace, which is through-flowed by plasma (“plasma flame” or “plasmabeam”), is widened due to the change and is widened to a length of up to2.5 m. The geometric extent of the plasma leads to a uniform widening—a“defocusing”—and to an acceleration of a process beam which is injectedinto the plasma with a feed gas. The material of the process beam whichis dispersed in the plasma to a cloud and is partially or completelymelted there arrives evenly distributed at the surface of the substrate10.

The plasma spray apparatus 1 illustrated in FIG. 1 includes a plasmagenerator 3 known per se having a non-closer illustrated plasma torchfor the generation of a plasma. In a manner known per se a process beam2 is generated by the plasma generator 3 from a starting material P, aprocess gas or a process gas mixture G and electrical energy E. Theintroduction of these components E, G and P is symbolized in FIG. 1 bythe arrows 4, 5, 6. The generated process beam 2 exits through an exitnozzle 7 and transports the starting material P in the form of a processbeam 2, in which the material particles 21 are dispersed in the plasma.This transport is symbolized by the arrow 24. The material particles 21are generally powder particles. The morphology of the layer 11 depositedon the substrate is dependent on the process parameters and inparticular on the starting material P, the process enthalpy and thetemperature of the substrate 10. The plasma generator 3 and/or theplasma torch are preferably pivotable with regard to the substrate 10 asis indicated by the double arrow A in FIG. 1. Thus, the process beam 2can be moved to and fro over the substrate 10 in a pivot movement.

It can be determined for how long and how frequently a certain point onthe substrate 10 is impinged by the process beam without interruption bythis to and fro movement. The longer this period of time is or the morefrequent this point is impinged, the higher the temperature of thesubstrate 10 is at this point. The to and fro movement, as well as theother process parameters are set so that a temperature of the substrate10 amounts to between 250 and 850° C. The process parameters requiredfor this are determined in particular in test runs. Furthermore, theprocess parameters are set so that the temperature of the substrate isonly larger than 800° C. for at most 5 minutes. Thus, on the one hand,the formation of a chromium oxide layer and, on the other hand,so-called creeping are prevented or only enabled to a minimum degree.

The porous substrate 10 is manufactured from an iron alloy by sinteringwhich has a chromium proportion which is larger than 25 weight percent.The substrate is composed, for example, of an alloy composed of 47%nickel (Ni), 22% chromium (Cr), 18% iron (Fe), 9% molybdenum (Mo), 1.5%cobalt (Co), 0.6% tungsten (W), 0.1% carbon (C), 1% manganese (Mn), 1%silicon (Si) and 0.008% boron (B).

The substrate has a useful porosity of 30% with respect to an overallvolume of the substrate.

In this connection the starting material P is injected into a plasmadefocusing the material beam and is partially or completely meltedtherein or at least made plastic at a low process pressure which is atmost 10 000 Pa and preferably at least 50 Pa and at most 2000 Pa for thedescribed LPPS process. For this purpose a plasma having a sufficientlyhigh specific enthalpy is generated so that a very thin and dense layer11 arises on the substrate. The variation of the structure issignificantly influenced and can be controlled by the coatingconditions, in particular the process enthalpy, the work pressure in thecoating chamber as well as the process beam. Therefore, the process beam2 has properties which can be determined by the controllable processparameters.

For the manufacture of the oxygen permeable membrane, the layer 11 isgenerated so that it has a very dense microstructure.

First of all the method step of generating the layer 11 by means of LPPSwill be explained in detail.

A powder having a suitable composition is selected as a startingmaterial P as will still be explained further on in detail. As alreadymentioned the plasma flame is already very long during the LPPS methoddue to the set process parameters in comparison to conventional plasmaspray. Furthermore, the plasma flame is very strongly widened. A plasmawith a high specific enthalpy is generated, whereby a high plasmatemperature results. A very high introduction of energy into thematerial particles 21 is brought about through the high enthalpy and thelength and/or the size of the plasma flame, which material particles 21are thereby, on the one hand, strongly accelerated and, on the otherhand, brought to a very high temperature so that they melt on very welland are also very hot after their deposition on the substrate 10. Since,on the other hand, the plasma flame and thus the process beam 2 are verystrongly widened, the local heat flow in the substrate 10 is so low sothat a thermal damage of the material is avoided. It is in particularavoided that the porous structure of the substrate 10 is damaged at theboundary to the layer 11 which would influence the capability of use ofthe layer 10 as a membrane. The widened plasma flame further has theresult that, typically on the one time covering of the substrate 10 withthe process beam 2, the material particles 21 are deposited in the formof individual splats which do not yet generate a continuous layer, thismeans a connected layer. Thereby very thin layers 11 can bemanufactured. The high kinetic and thermal energy which the materialparticles obtain during their long stay in the plasma flame incomparison to conventional plasma spray methods facilitate the formationof a very dense layer 11 which in particular has few boundary layerhollow spaces between splats lying on top of one another.

The plasma is, for example, generated in a plasma torch known per se inthe plasma generator 3 having an electric direct current and isgenerated by means of a pin cathode as well as a ring-shaped anode. Thepower consumption of the plasma torch lies in the region of up to 180kW. The power supplied to the plasma, the effective power, can bedetermined empirically with regard to the resulting layer structure. Theeffective power which is provided by the difference between the electricpower and the heat dissipated by cooling from experience lies, e.g. inthe range of 40 to 130 kW, in particular 80 to 100 kW. For this purpose,it has been tried and tested when the electric current for thegeneration of the plasma lies between 1000 and 3000 A, in particularbetween 1500 and 2600 A.

A value of between 10 and 10000 Pa, preferably of between 50 and 2000 Pais selected for the process pressure of the LPPS-TF plasma spraying forthe generation of the oxygen permeable membrane in the process chamber12.

The starting material P is injected into the plasma as powder material.

The process gas for the generation of the plasma is preferably a mixtureof inert gases, in particular a mixture of argon Ar, helium He andpossibly hydrogen H. In practice, the following gas flow rates for theprocess gas have been tried and tested in particular:

Ar flow rate: 30 to 150 SLPM, in particular 50 to 100 SLPM;

H₂ flow rate: zero to 20 SLPM, in particular 2 to 10 SLPM;

He flow rate: zero to 150 SLPM, in particular 20 to 100 SLPM,

with the overall flow rate of the process gas preferably being smallerthan 200 SLPM and in particular amounting to 100 to 160 SLPM.

It can be advantageous when the substrate is moved—additionally oralternatively—during the material deposition by means of rotationalmovement or pivotal movements relative to this cloud.

In the following, reference is made to the example particularly relevantfor practice in which the oxygen permeable membrane is composed of aceramic which beside oxygen includes the elements lanthanum (La),strontium (Sr), cobalt (Co) and iron (Fe). Such ceramics are referred toas LSCF. In this connection, it is driven for that the membrane shouldbe composed as completely as possible of a perovskite structure.However, it is naturally understood that the invention is not limited tosuch substances, but in particular it is also suitable for other ceramicmaterials, specifically oxides of the perovskite type.

As already mentioned the starting material P is provided in the form ofa powder. The plasma spray method is then carried out so that thechemical composition of the layer is at least substantially the same asthe chemical composition of the starting material.

LSCF as a ceramic material belongs to the oxides of the perovskite typewhich substantially have the form ABO₃. In this connection A representsLa_(x)Sr_(1-x) and B represents Co_(y)Fe_(1-y). However, it should benoted that the stoichiometry must not necessarily be satisfied exactly.It is certainly possible that the La content and the Sr content and/orthe Co content and the Fe content do not exactly match to one. Also theoxygen content can deviate from the precise stoichiometry. For thisreason, it is common to state the oxygen content with 3-σ, with σ beingthe deviation of the oxygen content from the stoichiometric equilibriumweight. The minus sign indicates that this deviation is generally anoxygen deficiency, this means that the oxygen is presentunder-stoichiometrically.

In the example described here LACF is present in the form ofLa_(0.58)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-σ). The starting material P ispresent as powder. Different methods can be used for the manufacture ofthe powder particles: for example, spray drying or a combination ofmelting and subsequent breaking and/or grinding of the solidified melt.

The manufacture of such powders is known per se and does not require adetailed explanation in this context. In view of the plasma spraying itis preferred when the powder seeds, for example, have a size of 25±5 μm.

The value of σ for the deviation of the oxygen content from thisstoichiometry, for example, amounts to 0.3.

For the two examples described in the following,La_(0.58)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-σ) is respectively used as thestarting material. The process pressure in the process chamber 12 is setto a value between 50 and 2000 Pa. A plasma beam or a process beam 2 ofhigh enthalpy is generated by means of a plasma torch which can generatea plasma of high specific enthalpy of up to 10,000 to 15,000 kJ/kg andwhich consumes a power of up to 180 kW. The process beam 2 has a lengthof 1000 to 2000 mm and a diameter of up to 200-400 mm. The length of theprocess beam 2 substantially corresponds to the spray distance, thismeans the distance D between the exit nozzle 7 and the substrate 10. Aporous plate of a high temperature nickel-based alloy or of a refractoryceramic serves, for example, as a substrate. The starting material P isintroduced by means of two powder supplies, with the feed rate being 120g/min, typically amounting to 40 g/min. By means of a pivot movement ofthe plasma torch a thin and dense layer 11 is applied onto the substrate10, with the high introduction of energy in the material particles 21and the high (ultrasonic) velocity in the process beam 2 enabling a verydense structure of the layer 11. The layer 11 is sprayed up until it hasan overall thickness of 20-60 gm. The coating time amounts toapproximately one minute. During the thermal spraying oxygen is suppliedto the process chamber 12 and indeed having a flow rate of at least 1%,preferably of at least 2% of the overall flow rate of a process gas.Hereby the reduction and the degradation of the starting material P orone of its components is avoided or at least strongly reduced. Thedeposition or the separation of elemental Co or Fe or their connectionsis avoided or at least strongly reduced. From this, it results that thechemical composition and the phase composition of the layer 11 issubstantially the same as that of the starting material P.

EXAMPLE 1

The process is carried out as is described above. The mixture of argonand helium is used as a process gas, with the Ar flow rate amounting to80 SLPM and with the He flow rate amounting to 40 SLPM, so that theoverall flow rate of the process gas amounts to 120 SLPM. The currentfor the generation of the plasma amounts to 2600 A.

EXAMPLE 2

The process is carried out as is described above. A mixture of argon,helium and hydrogen is used as a process gas, with the Ar flow rateamounting to 80 SLPM, the He flow rate amounting to 20 SLPM and with theH₂ flow rate amounting to 6 SLPM, so that the overall flow rate of theprocess gas amounts to 106 SLPM. The current for the generation of theplasma amounts to 2600 A.

In both cases oxygen permeable membranes result whose chemicalcomposition and perovskite phase structure is substantially the same asthat of the starting material.

A layer 11 acting as an oxygen permeable membrane on a porous substrate10 is very schematically illustrated in FIG. 2. The substrate has pores30 which are uniformly distributed in the substrate 10 and are thusconnected amongst one another so that the substrate 10 is gas-permeable.The mentioned connections cannot be seen in FIG. 2 since this is a verysimplified illustration, on the one hand, and, on the other hand, onlyis a section in a plane, the pores are, however, naturally arranged inthree dimensions. The layer 11 is arranged at a coating surface 31 ofthe substrate 10. A pressurized gas mixture contacts at a back substrateside 32 lying opposite of the coating surface 31 from which mixture theoxygen should be extracted. The gas mixture is symbolized by the arrow33. The gas mixture is transported to the coating surface 31 and thus tothe layer 11 via the pores 30 connected amongst one another. Due to thepermeability of the layer 11 for oxygen ions these exit through thelayer 11 and finally combine to oxygen molecules O₂. Thereby the oxygenis extracted from the gas mixture.

A section of the substrate 10 and the layer 11 is shown in a detailedview in FIG. 3. In this connection pores 30 open to the layer 11 areillustrated which likewise are connected to other pores. The pores 30are formed between metal particles 34 which are combined by sintering.It can clearly be seen in FIG. 3 that the gas mixture can be guided tothe layer 11 through the substrate 10.

In FIG. 4 a very schematic section of the substrate is illustrated in atop view. The pores 30 again form between the metal particles 34. Aportion of an overall pore surface of an overall surface of the coatingsurface in this connection amounts to approximately 40%, this means thatthe portion of the nonhatched area to the overall surface of theillustrated rectangle amounts to approximately 40%. The mean pore sizeof the pores 30 in this connection amounts to at least 1 micrometer.

In FIG. 5 a substrate 110 having a layer 111 is illustrated. In order toimprove the supply of the gas mixture to the layer 111 the substrate 110has micro-passages 140 which can have a diameter of between 5 and 150micrometers. The micro-passages 140 reach from a back substrate side 132in the direction of a coating surface 131, wherein they end prior toarriving at the coating surface 131. The micro-passages 140 areintroduced by laser drilling either before or after the coating.

1. A plasma spray method for the manufacture of an ion conductingmembrane, in particular of a hydrogen ion conducting membrane or of anoxygen ion conducting membrane, wherein the membrane is deposited as alayer (11, 111) on a substrate (10,110) in a process chamber (12),wherein a starting material (P) is sprayed onto a coating surface (31,131) of the substrate (10, 110) by means of a process gas (G) in theform of a process beam (2), wherein the starting material is injectedinto a plasma at a low process pressure which is at most 10000 Pa and ispartially or completely melted there, characterized in that thesubstrate (10, 110) has pores (30) which are connected amongst oneanother so that the substrate (10, 110) is gas permeable and a portionof an overall pore area of an overall area of the coating surface (31,131) amounts to at least 30%, in particular to at least 40%.
 2. A plasmaspray method in accordance with claim 1, characterized in that the pores(30) have a mean pore size of at least 1 micrometer.
 3. A plasma spraymethod in accordance with claim 1, characterized in that the substrate(10, 110) has a useful porosity of at least 20%, in particular of atleast 30% with respect to an overall volume of the substrate (10, 110).4. A plasma spray method in accordance with claim 1, characterized inthat micro-passages (140) are introduced into the substrate (110) beforeor after the coating for the improvement of a gas flow possibility inthe direction of the coating surface (131).
 5. A plasma spray method inaccordance with claim 4, characterized in that the micropassages (140)are orientated in the direction of the coating surface (131) from a rearsubstrate side (132) lying opposite the coating surface (131) and endbefore the coating surface (131).
 6. A plasma spray method in accordancewith claim 1, characterized in that the substrate (10, 110) ismanufactured from an iron alloy which has a portion of chrome which islarger than 20 weight percent and in particular is larger than 25 weightpercent.
 7. A plasma spray method in accordance with claim 1,characterized in that process parameters are set so that a temperatureof the substrate (10, 110) amounts to between 250 and 850° C. during thecarrying out of the method.
 8. A plasma spray method in accordance withclaim 7, characterized in that process parameters are set so that atemperature of the substrate (10, 110) is higher than a temperatureboundary of in particular 800° C. only for a maximum period of time, inparticular of 5 minutes, during the carrying out of the method.
 9. Aplasma spray method in accordance with claim 1, characterized in thatthe layer (11, 111), which forms the membrane, is composed of a ceramicmaterial which is an oxide of the perovskite type.
 10. A plasma spraymethod in accordance with claim 9, characterized in that the layer (11,111) is composed of a perovskite which includes lanthanum (La),strontium (Sr), cobalt (Co), iron (Fe), chromium (Cr), titanium (Ta),barium (Ba), zirconium (Zr), cerium (Ce), yttrium (Y), ytterbium (Yb),europium (Eu) or aluminum (Al).
 11. A plasma spray method in accordancewith claim 1, characterized in that the layer (11, 111) generated on thesubstrate (10, 110) has a thickness of less than 150 micrometers.
 12. Aplasma spray method in accordance with claim 1, characterized in that anoverall flow rate of the process gas is smaller than 200 SLPM and inparticular amounts to 100 to 160 SLPM.
 13. A plasma spray method inaccordance with claim 1, characterized in that the process gas is amixture of argon and helium.
 14. A plasma spray method in accordancewith claim 1, characterized in that the process beam (2) is pivoted orrastered relative to the coating surface (31, 131) of the substrate. 15.An ion conducting membrane, in particular a hydrogen ion conductingmembrane or an oxygen ion conducting membrane on a substrate (10, 110)which is deposited as a layer (11, 111) on the substrate (10, 110) witha plasma spray method in a process chamber (12), wherein a startingmaterial (P) is sprayed onto a coating surface of the substrate (10,110) by means of a process gas (G) in the form of a process beam (2),wherein the starting material is injected into a plasma at a low processpressure which is at most 10000 Pa and is partially or completely meltedthere, characterized in that the substrate (10, 110) has pores (30)which are connected amongst one another so that the substrate (10, 110)is gas permeable and a portion of an overall pore surface of an overallsurface of the coating surface (31, 131) amounts to at least 40%.